(1) Field of the Invention
The present invention relates to an electrophotographic toner and an electrophotographic apparatus, and in particular relates to an electrophotographic toner which has fine particles externally added so as to provide an abrading effect and refresh the photoreceptor surface well as relating to an elect:rophotographic apparatus having an efficiently functional developing system.
(2) Description of the Prior Art
Conventionally, the electrophotographic technique, the application of the Carlson process, has been widely used for image forming using the toner. An apparatus using the Carlson process typically uses a photoreceptor drum having a photosensitive layer on the surface thereof and has a charger, an exposing device, a developing device, a transfer device, a fixing device, a cleaner and an erasing device, all arranged around the photoreceptor drum in the mentioning order. Now, the Carlson process will be explained. In this process, first in the dark place, the photoreceptor drum surface is uniformly charged by the charger. Then, the exposing device illuminates the photoreceptor drum surf ace with a light image of a document so as to release charge at the areas which have been illuminated with light, thus forming an electrostatic latent image on the photoreceptor drum surface.
Next, the toner which has been charged with a polarity that is opposite to the electric field of charge on the photoreceptor drum adheres to this static latent image so as to develop the static latent image into a visual image. Thereafter, a recording material such as paper is laid over the visual image while charge with a polarity opposite to the toner is given to the rear side of the recording material by corona discharge from the transfer device, whereby the toner image is transferred to the recording material. After the transfer step, the toner image is fixed to the recording material by the heat and pressure from the fixing device, forming a permanent image.
The leftover toner that has not transferred to the recording material and remains on the photoreceptor drum is removed by the cleaner. The static latent image on the photoreceptor drum is charge erased by the erasing device. Thereafter, the above process, starting with charging of the photoreceptor drum, is repeated so as to effect continual image forming. The toner to be used for the electrophotocraphic technology using the above-mentioned Carlson process is a coloring powder forming visual images and also has the charge function and the fixing function to recording materials. Further, the toner is given with various functions, depending upon the properties of the apparatus to which the toner is used.
As one of the functions of the toner, the toner performs the function of abrading the photoreceptor and refreshing the photoreceptor surface when the toner remaining on the photoreceptor surface after the transfer stage is removed. This function is mainly attributed to inorganic particles externally deposited over the toner surface. In order to make the function of refreshing the photoreceptor surface more efficient, Japanese Patent Application Laid-Open Hei 8 No.137124 discloses a technique for uniformly dispersing the abrasive over the toner surface by preliminary mixing of the abrasive and a fluidizer and then blending them with the toner particles by a large-scale mixer.
However, if fine particles having abrading effect are externally added to the toner by this method, the particles themselves may firmly Stick to the photoreceptor surface and produce image deficiencies.
The present invention has been devised in view of the above drawbacks and it is therefore an object of the present invention to provide an electrophotographic toner which provides the function of refreshing the photoreceptor surface without causing any damage to the images and also provide an electrophotographic apparatus having a developing system providing the optimized effective function.
The inventors hereof have made various investigations in order to solve the above drawbacks and have found out the new fact that a toner can produce the function of refreshing the photoreceptor surface without causing any deficiency by limiting the volume mean diameter, specific surface area and the mixing ratio of the fine inorganic particles eternally added and adhering to the toner to associated specified ranges and thus have completed the invention.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, an electrophotographic toner constituting a dual component developer in combination with a powdered magnetic carrier, comprises:
toner base particles made up of a binder resin containing a coloring agent and the like with a fluidizer and other particles adhering to the surface thereof; and
fine inorganic particles having a volume mean diameter of 250 to 600 nm (i.e., 0.25 to 0.6 xcexcm) with a specific surface area of 0.25 to 0.5 m2/g for serving as an abrasive, and is characterized in that 1.5 to 2 parts by weight of the abrasive is externally added to 100 parts by weight of the toner base particles and made to be deposited over the toner base particle surface.
In accordance with the second aspect of the present invention, the electrophotographic toner having the above first feature is characterized in that the fine inorganic particles are made to be externally deposited over the toner base particle surface by blending the two components in a mixer for 30 to 90 seconds with the mean linear speed of the mixer""s agitator blade set at 600 m/min to 900 m/min.
In accordance with the third aspect of the present invention, an electrophotographic apparatus for performing an electrophotographic process using a dual-component developer made up of a toner and a powered magnetic carrier, comprises: a photoreceptor cleaning device is located above the level that passes through the center of the photoreceptor drum, and is characterized in that the electrophotographic toner constituting a dual component developer in combination with a powdered magnetic carrier, comprises: toner base particles made up of a binder resin containing a coloring agent and the like with a fluidizer and other particles adhering to the surface thereof; and fine inorganic particles having a volume mean diameter of 250 to 600 nm (i.e., 0.25 to 0.6 xcexcm) with a specific surface area of 0.25 to 0.5 m2/g for serving as an abrasive, and that 1.5 to 2 parts by weight of the abrasive is externally added to 100 parts by weight of the toner base particles and made to be deposited over the toner base particle surface.
In accordance with the fourth aspect of the present invention, the electrophotographic apparatus using an electrophotographic toner, having the above third feature, is characterized in that the fine inorganic particles are made to be externally deposited over the toner base particle surface by blending the two components in a mixer for 30 to 90 seconds with the mean linear speed of the mixer""s agitator blade set at 600 m/min to 900 m/min.
In accordance with the fifth aspect of the present invention, an electrophotographic apparatus for performing an electrophotographic process using a dual-component developer made up of a toner and a powered magnetic carrier, comprises:
a photoreceptor drum having a diameter of 30 to 40 mm and rotating at a linear speed of 80 mm/s or higher, and is characterized in that the electrophotographic toner constituting a dual component developer in combination with a powdered magnetic carrier, comprises: toner base particles made up of a binder resin containing a coloring agent and the like with a fluidizer and other particles adhering to the surface thereof; and fine inorganic particles having a volume mean diameter of 250 to 600 nm (i.e., 0.25 to 0.6 xcexcm) with a specific surface area of 0.25 to 0.5 m2/g for serving as an abrasive, and that 1.5 to 2 parts by weight of the abrasive is externally added to 100 parts by weight of the toner base particles and made to be deposited over the toner base particle surface.
In accordance with the sixth aspect of the present invention, the electrophotographic apparatus using an electrophotographic toner, having the above fifth feature is characterized in that the fine inorganic particles are made to be externally deposited over the toner base particle surface by blending the two components in a mixer for 30 to 90 seconds with the mean linear speed of the mixer""s agitator blade set at 600 m/min to 900 m/min.
The embodiments of the present invention will hereinafter be described in detail.
The toner base particles to be handled in the present invention may be of publicly known toner particles which have been conventionally used in the dry development system Such toner particles are produced by dispersing a coloring agent and other additives in a fixer resin. Here, various types of thermoplastic resin can be employed as the fixer resin, but acrylic polymers and styrene-acrylic copolymers are preferably used and in particular styrene-acrylic copolymers are most preferable.
As the coloring agent dispersed in the fixer resin, carbon black, alloy azo-dyes, other oil-based dyes and pigments are known and can be used as appropriate. An amount of 1 to 30 parts, preferably 2 to 20 parts by weight of such coloring agents is added to 100 parts by weight of the fixer resin. Additives other than the coloring agents include, for example, charge control agents, anti-offset agents, etc.
Here, the charge control agent is blended in order to control the tribo-electrification performance of the toner. There are two types of charge control agents, the positive charge control type and the negative charge control type. The anti-offset agent is blended in order to provide anti-offset effect for the toner. A variety of anti-offset agents have been known and can be used as appropriate. Typically, an amount of 0.1 to 10 parts, preferably 0.5 to 8 parts by weight of a charge control agent is added to 100 parts by weight of the fixer resin while an amount of 0.1 to 10 parts, preferably 0.5 to 8 parts by weight of an anti-offset agent is added to 100 parts by weight of the fixer resin.
The toner base particles to which the present invention is applied are prepared by fusing and kneading, for example, the fixer resin, coloring agent and other additives, and then crushing the resultant material by crushing and classification of the obtained particles. The toner base particles used in this case should have a mean particle size of 5 to 15 xcexcm, preferably 7 to 12 xcexcm. Usually, a fluidizer such as silica, titanium oxide, alumina or the like is deposited on the surface of the toner base particles.
Particularly, silica is the most preferable in view of improving the fluidity. The fluidizer used in this case should have a mean particle size of 0.1 xcexcm or below, preferably 0.005 to 0.05 xcexcm, so that its mean particle size is much smaller than that of the abrasive made up of fine inorganic particles described hereinbelow.
In the present invention, the timing when fluidizer is made to be deposited on the toner base particle surface should not be limited. For example, the fluidizer may be uniformly mixed beforehand with the toner base particles in a mixer so as to uniformly adhere thereto and then the aftermentioned abrasive is made to be externally deposited. Alternatively, the abrasive and the fluidizer may be mixed altogether with the toner base particles so that the fluidizer as well as the abrasive will be made to be externally deposited over the toner base particle surface.
Examples of the abrasive used in the electrophotographic toner of the present invention include fine inorganic particles of magnetite, alumina, cerium oxide, strontium titanate, etc., and magnetite, alumina and the mixture thereof are particularly preferred.
Here in the present invention, it is essential that the abrasive should have a volume mean diameter of 250 to 600 nm, preferably 290 to 550 nm, with a specific surface area of 0.25 to 0.5 m2/g, preferably 0.29 to 0.5 m2/g and that 1.5 to 2 parts by weight of the abrasive should be externally added to 100 parts by weight of the toner base particles and made to be deposited over the toner base particle surface.
Here, when the particle size (volume mean diameter) of the fine inorganic particles to be externally deposited is smaller than the above specified value, such particles of smaller diameters are liable to drop off the toner base particle surface because the surface area per grain of a smaller particle is smaller compared to that of a greater particle and the contact area with the toner base particle becomes smaller. The fine inorganic particles having fallen off are pressed against the photoreceptor drum by the cleaning blade and caused to adhere to the photoreceptor drum surface when it scrapes the untransferred toner from the photoreceptor drum surface. As a result, deficiencies appearing on the rotational cycle of the drum would occur in the image. On the other hand, with the particle size (volume mean diameter) of the fine inorganic particles greater than the above specified value, even when some part of the fine inorganic particles has fallen off the base particles, the fine inorganic particles not having fallen off and remaining deposited on the toner surface will provide the scraping function so as to reduce the adhesion of the fine inorganic particles to the drum.
With the volume mean diameter of the particles specified within the constant range, the particles will have more irregularities on the surface thereof as they have a greater specific surface area than the aforementioned specified value. As the fine inorganic particles externally added as the abrasive have more irregularities on the surface thereof, the function of abrading the photoreceptor drum surface become stronger. Accordingly, the function is effective enough in scraping the fine inorganic particles having fallen from the toner surface and becoming deposited on the photoreceptor drum surface, whereby it is possible to prevent image defect:s occurring due to adherence of the fine inorganic particles onto the photoreceptor drum. However, if the specific area becomes too large beyond the above specified value, the abrading function decreases, so that the fine inorganic particles adhering to the drum cannot be scraped, producing an unwanted result.
Next, when the fine inorganic particles as the abrasive is externally added in an amount lower than the aforementioned specified value, the fine inorganic particles which have fallen from toner base particles and adhered to the photoreceptor drum becomes less in number. Accordingly, the possibility of occurrence of image defects due to filming becomes smaller. However, the originally intended function as the abrasive (the functions of scraping the deposit on the drum and refreshing the drum surface) is also degraded. Further, the fine inorganic particles externally added as the abrasive have the function of scraping the fine inorganic particles which have fallen from the toner base particles and adhered to the photoreceptor drum, from the photoreceptor drum, by their own abrading force. Therefore, when the added amount increases and exceeds a certain level, image deficiencies due to filming decrease.
Since the fine inorganic particles externally added as the abrasive is conductive, they functions to leak electric charge from the toner. Therefore, if the fine inorganic particles are added in excess of the aforementioned specified value, the quantity of charge on the toner decreases, causing deficiencies such as toner scatter, fogging and the like.
Thus, the above problems can be solved by specifying the abrasive used in the present invention so as to meet both requirements of the volume mean diameter being 250 nm or greater, preferably 290 to 550 nm and the specific surface area being 0.25 to 0.5 m2/g, preferably 0.29 to 0.5 m2/g, and blending 1.5 to 2 parts by weight of the abrasive with 100 parts by weight of the toner base particles, so that the abrasive is made to be externally deposited on the toner base particles.
In the present invention, at the stage of externally adding the abrasive to and mixing it with the toner, blending in the mixer is performed for 30 to 90 seconds with the mean linear speed of the mixer""s agitator blade set at 600 m/min to 900 m/min.
Here, when external addition and blending is performed in a high-shear field which can be obtained by enhancing the linear speed of rotation of the agitating blade of the mixer, it is possible to give strong impacts between the external additive and the base particles when they collide against each other in the mixer. As a result, it is possible to cause the fine inorganic particles to firmly adhere to the toner base particle surface. Therefore, it is possible to prevent the fine inorganic particles from dropping off in a more effective manner if external addition is performed by high-speed agitation.
If the shear force during external addition and blending is too strong, the fine inorganic particles externally added as the abrasive are forced to become embedded into the interior of the base particles, so that the originally intended function as the abrasive (the functions of scraping the deposit on the drum and refreshing the drum surface) is degraded. This means that too much enhancement of the rotational speed of the agitator blade in the mixing device is not favorable. That is, there is an optimal operating range of the linear speed of rotation of the agitator blade of the mixer in order to produce a toner having the effect of abrading the photoreceptor without causing any harmful effect. As a result of the investigation in this view point, the optimal operating range at the stage of externally adding the abrasive to and mixing it with the toner, has been found that blending in the mixer is performed for 30 to 90 seconds with the mean linear speed (the speed of the mid blade) of the agitator blade set at 600 m/min to 900 m/min.
Next, in a dual-component development electrophotographic process using the toner and a powdered magnetic carrier as in the present invention, when the copier or printer has a photoreceptor cleaning device (unit) arranged above the level passing through the center of the photoreceptor drum, the untransferred toner remaining on the photoreceptor drum is scraped by the cleaning blade and then the scraped toner particles move in contact with the drum surface as they are conveyed to a collecting box. Accordingly, the collected toner comes into contact with the photoreceptor drum more often compared to a configuration where the photoreceptor cleaning device is located in the lower position of the photoreceptor, and hence the fine inorganic particles having fallen from the base particles become likely to adhere to the photoreceptor drum. Further, this arrangement is also more advantageous in saving space, constituting a used toner recycling system, and designing the other apparatus configuration. Therefore, the electrophotographic toner of the present invention as specified above is particularly effective when used in such an electrophotographic apparatus.
From the viewpoint of space saving of the system, a copier/printer having a small-diametric photoreceptor drum that rotates at a higher speed is advantageous. However, the untransferred toner tends to pass through the nip between the cleaning blade and the photoreceptor surface. To deal with this, it is necessary to enhance the squeezing pressure of the cleaning blade. This causes the toner held between the blade and the photoreceptor drum to be stressed greatly, the fine inorganic particles on the base particle surface are liable to drop off. However, it is particularly effective when the above-described electrophotographic toner of the present invention is used in an electrophotographic apparatus having a photoreceptor drum having a diameter of 30 to 40 mm with its linear velocity set at 80 mm/s or greater.